This invention relates to an improved method of cooling board material of inorganic substance, such as calcium silicate or asbestos-cement, after aging of the board material in an autoclave.
In aging board material comprising calcium silicate, Tobemonite or Zonotlite is generated from exothermic hydration reaction between silicic acid family compound and lime family compound. It is necessary to age the board material under high temperature and high pressure, for example, using saturated steam of 160.degree. C., in for example an autoclave. When the board material is of asbestos-cement, the aging can be done in an autoclave, and the Tobermonite crystals are formed by exothermic hydration reaction between compounded raw materials of silica sand and cement.
When using an autoclave for aging treatment, a number of unaged boards are piled within the autoclave and then the autoclave is heated using saturated steam of high temperature and high pressure. At the end of the aging step, the temperature of the aged board is so high (e.g. 150.degree. C.) that in order for the boards to be further treated, they must first be cooled properly. This cooling was done in the prior art by introducing outdoor atmospheric air into the autoclave by opening the cover thereof.
However, disadvantageously, in this type of cooling, cracks are easily generated when the heat is suddenly released from the board upon contact with the outer atmosphere. Also, it is difficult to keep the boards in a dried state after cooling. Generally, the aged board has numerous tiny pores. During the reduction of pressure in the autoclave, pressure within the pores is also reduced. Steam is generated in the pores, but cannot be readily diffused. Thus, in the prior art, a separate drying room is used. This is expensive in terms of capital expenditure for capital equipment and for added step of drying and manpower required to maintain the equipment. Furthermore, the autoclave used has a cover on one end. Thus, when atmospheric air is introduced, cooling is not uniform throughout the autoclave, and the board materials will cool at different rates depending upon their location in the autoclave. To compensate, cooling is done for a longer period of time.